Cases of Paris title from hillside are very -- and this case in particular was extremely unusual and terrific and that -- -- in 2003. An unexpected letter was mailed to northwestern university -- -- psychology professor Robert -- From an inmate at a maximum security prison. There was extremely unusual first letter was not was not long. It was focused on his interest in trying to understand. Anti social personality disorder. The letter was from convicted murderer Tom portal. Hamlin who specializes in forensic -- psychology. Had once conducted a psychological assessment on the death row inmate. That was before all little and all other Illinois death row inmates have their sentences commuted by then governor George Ryan. Since his -- had been changed from death to life without parole he was. No motivated and very interested in trying to understand why he -- committed. The crime. This this massive horrible crimes that he had he had committed. In November of 1985. When he was just eighteen Tom -- -- brutally murdered his family in their home one by one. First his father Bob -- leader his mother Caroline and lastly his three siblings Sean and Robin and Scott. Crime rocked the town of Mount Vernon Illinois and the nation. This was a crime meant that time that was covered by lead to New York Times covered by the Chicago Tribune covered by. All the major television networks. He confessed to the murders and that his trial was painted as a troubled teen. Who is about to be kicked out of the house when he went on a drug fueled Rampage killing his entire family. But also the trial documented record of sadistic physical and mental abuse by his mother surfaced. The abuse was directed at Tom and his -- show on it seems their father knew about it but did nothing to stop one abusive. Parent one parent is an enabler that happens every day. Bank and the unfortunate recipients of that abuse often don't. You know kill anybody your commit any crimes. To better understand would -- -- to commit such a heinous crime. Amylin agreed to guide him through -- therapeutic exercise. In the form of waters after -- offered. To provide him with some framework and questions and guidelines. Four were. This. Process of self exploration and analysis. I then began receiving. Big -- -- di tale copious. Letters describing me. In painful -- -- about his childhood. But his relationship with his mother. By the end of the unconventional therapy intervention -- came to the conclusion that a combination of his mother's abuse. His father's enabling an old -- anti social personality disorder. Set the stage for there. Four. Cave basically. Devastating. -- Tragic event to occur Hanlon says there are lessons to be learned from horrible story and that's why he published the book survived by one. The light and mind of -- family mass murderer. -- have been prevented yes that is that's really the objective of this book is two try to increase awareness. Educate people. So that they can identify these behaviors and these features in advance and prevent horrible tragic. Domestic homicides like this.

This transcript has been automatically generated and may not be 100% accurate.

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